For example, there is known a process and apparatus for continuous casting of hollow ingots (of. FRG Pat. No. 804,840, cl. B 22d, 11/06, 1951).
The process of the patent referred to above includes molten metal being continuously delivered through a bottom gate into a mold with an open top end. The skin of metal solidifying at the interior surface of the mold is pulled upwardly. The solidified metal skin forms the ingot body.
There is also known a freezing-out process for continuous casting of hollow ingots and an apparatus for effecting same (of, "Calculation of Ingots", by A. I. Vejnik, Moscow Publishers, Engineering, 1964, pp. 239-261; "Thermodynamics of a Casting Mold", by A. I. Vejnik, Moscow Publishers, Engineering, 1968, pp. 252-264; and "Chill Mold", by Vejnik, Moscow Publishers, Engineering, 1972, pp. 127-137).
The aforementioned process is carried out with maximum rate of heat removal from the surface of solidifying ingot throughout the period of its formation in the mold. The withdrawal of the ingot is conducted continuously in a stepwise manner. An apparatus for carrying the continuous casting process into effect comprises a water-cooled mold having a working sleeve member with the interior surface thereof being cylindrical in shape, a coupling casing and a bottom gate through which molten metal is delivered into the interior of the mold.
The disadvantage of the known continuous casting processes and apparatus lies in the absence of a control system for regulating the intensity of heat removal the ingot within a mold. This, in turn, prevents from producing an ingot with a uniform thickness of its wall and with a prescribed structure and properties of its metal. Where pig iron is used for continuous casting, it is impossible to produce ingots free from hard spots and with a prescribed structure. Since the initial solidification of skin is effected at a vigorous heat dissipation, the crystallization of pig iron is accompanied by the formation of austenite dendrites and ledeburite eutectic. Vigorous heat dissipation or removal from the surface of solidifying ingot throughout the entire period of its formation (mean coefficient of heat removal is (2:3).multidot.10.sup.3 W/m.sup.2 .multidot.degrees), rules out decomposition of eutectic cementite. The resultant ingot has hard spots on its outer surface, with the structure of mottled iron in other parts thereof. Another serious disadvantage of the known processes is low stability of the continuous casting process per se. The reason for this is an enormous dynamic stress acting upon the solidifying initial skin of ingot during each cycle of its withdrawal. At the initial stage of its formation the skin has a high temperature ranging, for example during casting of pig iron, from 950 to 1050.degree. C. Tensile strength of pig iron at such temperatures is within the range of from 0.1 to 0.45 kg/mm.sup.2. Sharp increase in the ingot withdrawal speed in the beginning of each cycle results in the rupture of metal skin and, consequently, in the termination of the casting process.
It is, therefore, an object of the present invention to produce ingots featuring high quality metal.
Another object of the invention is to provide a process and apparatus for continuous casting of hollow ingots, whereby it will be possible to control the process of ingot formation by way of regulating the rate of heat removal from the continuous ingot.
These and other objects and features of the invention are accomplished by the provision of a process for the continuous casting of hollow ingots, comprising the steps of delivering molten metal through a bottom gate into the interior of a water-cooled mold and continuous withdrawing in an upward direction and in a stepwise manner the skin of solidified metal forming at the mold interior surface into an ingot, according to the invention, the rate, of heat emission from the surface of the ingot within the mold is changed gradually over the mold height from a given maximum value in the metal skin formation zone to a given minimum value at the emergence of the ingot from the mold, the speed of the ingot continuous withdrawal from the mold being increased gradually during each withdrawing cycle from zero to the maximum value for a length of time equal to a half the travelling time of the ingot, said speed being then gradually reduced back to zero for the same amount of time.
It is expedient that the rate of heat emission from the ingot at each point over the height of the mold be determined from the formula: EQU .alpha. = a(h/H).sup.b,
where
.alpha.- is the coefficient of heat emission from the ingot surface, (W/m.sup.2.deg); PA1 h - is the height of a mold, over which is determined the rate of heat emission, (m); PA1 H - is the overall height of the mold, (m); PA1 a - is an empirical coefficient depending upon the ingot material (W/m.sup.2.deg); PA1 b - is a nondimensional empirical coefficient depending upon the ingot material. PA1 V.sub.max - is the maximum value of the ingot withdrawal speed during its travelling cycle, (m/sec); PA1 A.sub.o - is the free coefficient of a Fourier series; PA1 A.sub.n - is the amplitude of oscillation of the corresponding harmonic component of the Fourier series; PA1 n - is the sequence of natural numbers 1,2,3,4 . . . ; PA1 .omega. = 2.pi./T is the pulsatance of the ingot travelling speed, (sec.sup.-1); PA1 T - is the oscillation period of the ingot travelling speed (sec); PA1 t - is the current time of the ingot withdrawal, (sec); PA1 .epsilon..sub.n - is the epoch angle of the corresponding harmonic component of the Fourier series, (rad); PA1 K - is the sequence of even numbers 0,2,4,6, . . . .
The speed of the ingot withdrawal from the mold during each withdrawing cycle can be determined from the formula: ##EQU1## where V.sub.t - is the current value of the ingot withdrawal speed during its travelling cycle, (m/sec);
The heat removal rate for pig iron can be regulated over the entire height of the mold, the empirical coefficients a and b being selected within the following range: EQU 200 .ltoreq. a .ltoreq. 2500 EQU -1.2 .ltoreq. b .ltoreq. -0.6
When steel is used for casting of hollow ingots, the rate of heat removal is usually regulated over the entire height of the mold, the empirical coefficients being selected within the following range: EQU 1000 .ltoreq. a .ltoreq. 1200 EQU -0.6 .ltoreq. b .ltoreq. -0.1
With nonferrous metals being used for casting of hollow ingots, the rate of heat removal is regulated over the entire height of the mold, the empirical coefficients being selected within the following range: EQU 300 .ltoreq. a .ltoreq. 10000 EQU -0.7 .ltoreq. b .ltoreq. -0.3
The aforesaid object of the invention is likewise attained in a continuous casting apparatus comprising a water-cooled mold connected through a coupling sleeve with a bottom gate system, a withdrawal-roll means for continuous upward pulling of an ingot, and a cut-off means for cutting the continuous ingot to lengths. According to the present invention, the mold is made in the form of an inveted truncated cone with the diameter of any base thereof being substantially less than its height, the upper portion of said mold being cylinder-shaped and formed at the interior surface thereof with a plurality of annular slots.
The diameter of projections formed by the slots should be slightly less than that of the upper base of the truncated cone, and the passages from projections to the slots should be made smooth.
It is advantageous that the apparatus of the present invention be provided with a detachable screen adapted to regulate the rate of the ingot cooling and mounted above the mold and in direct proximity to the ingot.
The provision of the screen makes it possible to regulate the rate of heat removal from the ingot and control the process of its formation, thereby producing continuous ingots of high-quality metal.